The present invention relates to integrated circuits, and more particularly, to a non-volatile memory with a regulated charge pump.
Semiconductor memories use peripheral circuit structures known as charge pumps which can generate voltages higher than that of the supply voltage of the integrated device in which the memory is formed. These voltages serve for writing, reading and erasure operations on the memory. A regulation system is generally associated with the charge pump for keeping its output voltage constant for a given range of currents absorbed by the load. Known regulation systems detect the output voltage of the charge pump, compare it with a constant reference voltage, and apply to the input of the charge pump a compensation signal based upon the comparison to keep the output at a predetermined voltage level.
The output of the charge pump is generally connected to the component that is to receive the voltage. This connection is provided by a connection element having electrical conduction characteristics which are dependent on temperature, design selections, and on the parameters associated with the manufacturing process. The voltage applied to the component is therefore not constant, as would be desirable, but depends on the temperature and on the above mentioned parameters. A method of avoiding this problem would be to pick up the voltage to be regulated directly downstream of the connection element but this is not possible in some cases, for example, in the case of an electrically erasable and programmable non-volatile memory, such as a flash EEPROM memory.
A typical EEPROM memory is formed on a substrate of semiconductor material as a matrix of memory cells each including a body region with p-type conductivity in which two (source and drain) regions with n-type conductivity are formed. These two regions are separated by a channel region. A floating gate electrode is disposed above the channel region, insulated therefrom by a thin layer of dielectric material. A control gate electrode extends above the floating gate electrode, insulated therefrom by another layer of dielectric material.
The cells of the matrix have their source regions connected together to a common terminal which, during programming and reading, is generally connected to the negative terminal (ground) of the voltage supply of the integrated circuit of which the memory is included therein. The drain regions of the cells of each column of the matrix are connected to one another by common connection lines, known as bit or column lines. The control gate electrodes of the cells of each row are connected to one another by common connection lines known as word or row lines.
Each cell of the matrix can be selected by a row decoder and a column decoder. Once selected, the cell can be biased by the application of suitable potentials to its terminals, and its state can be determined by a sense amplifier arranged in series with the respective bit line.
To write or program a cell of the memory, the bit line and the word line which identify it are brought to predetermined potentials higher than the common source potential, for example 5V and 9V, respectively, to cause hot electrons to pass through the thin dielectric layer from the substrate to the floating gate electrode. The electrons which accumulate in the floating gate electrode bring about an increase (e.g., 2-4V) in the threshold voltage of the cell.
To erase a cell, a positive potential (e.g., 5V) is applied to the common source terminal, a negative potential (e.g., xe2x88x928V) is applied to the word line (the control gate), and the bit line (the drain) is left floating. In these conditions, a strong electric field is developed between the floating gate electrode and the source region so that the negative charge formed by the accumulated electrons is extracted from the floating gate electrode by the tunnel effect. In a flash EEPROM memory, erasure takes place simultaneously for all of the cells of the matrix or for the cells of a selected section of the matrix.
To read a cell, the common source terminal is connected to ground, the bit line (drain) is brought to a positive potential, the word line (control gate) is brought to a higher positive potential, and the drain current is measured by the sense amplifier. An erased cell (logic level 1) conducts a relatively high current (e.g., 50 xcexcA) and a programmed cell (logic level 0) conducts a considerably lower current.
As mentioned above, a cell is selected by connecting its drain and control gate electrodes to biasing circuits outside the matrix by bit and word decoders. The decoders comprise, basically, electronic switches controlled by the system logic. The electronic switches are normally formed by MOS transistors having small dimensions. Since the resistance of these transistors when they are conducting is not negligible when the drain currents are high, as they are during the programming of a cell, the voltage drop in them is considerable. This would not be a problem if the voltage drop were constant and precisely predictable. However, this depends on the operating temperature and on the manufacturing parameters.
When the drain terminal of a cell is connected to a charge pump to program the cell, its potential therefore varies in an uncontrolled manner, even though the output voltage of the charge pump is stabilized. In particular, the voltage applied to the cell decreases as the temperature increases. Moreover, since the hot-electron emission mechanism becomes less efficient at high temperatures, the programming of a cell becomes problematic at high temperatures.
In view of the foregoing background, an object of the present invention is to provide a memory with a charge pump which does not have the problems as discussed above.
Another object of the present invention is to form a memory with a charge pump such that the voltage of a memory cell is in a predetermined biasing condition, for example, in the programming condition, and is independent of variations in temperature and in the manufacturing and design parameters of the memory.
These and other objects, advantages and features of the present invention are provided by a semiconductor memory comprising a plurality of electrically programmable, non-volatile memory cells connected to one another by row lines and by column lines to form a matrix of memory cells. A charge pump may be connected to the matrix of memory cells, and includes an input terminal, an output terminal, and a common terminal.
A regulator may be connected to the input of the charge pump for regulating an output voltage therefrom, and a plurality of controllable connection elements, with each controllable connection element connected between the output terminal of the charge pump and a respective column line. Control means selectively activates the plurality of controllable connection elements.
More particularly, the memory further includes a first element equivalent to a controllable connection element, and a second element equivalent to a memory cell in a predetermined biasing condition. The first equivalent element and the second equivalent element are connected in series with one another between the output terminal and the common terminal of the charge pump. The regulator may be connected between the second equivalent element and the input terminal of the charge pump for regulating the output voltage therefrom based upon a voltage present between terminals of the second equivalent element.